Device with an electrode, a spongiform permeable layer, an electrolyte and a means for applying a voltage

ABSTRACT

An apparatus having a percolating, sponge-like nanoparticulate layer, the apparatus being suitable for numerous technical applications. The object is accomplished by an apparatus having an electrode and a sponge-like, percolating, electrically-conductive layer comprising bound nanoparticles, the layer containing cavities and at least partially covering the electrode. The size of the nanoparticles is at most 15 times the size of a forming space-charge region in the layer; the cavities in the layer are at least partially filled with an electrolyte. The cavities of the layer are larger than the space-charge region in the electrolyte. The electrolyte also forms a film, which borders the surface of the layer located opposite the electrode. The apparatus also includes elements with which a voltage can be applied between the electrode and the electrolyte.

The invention relates to an apparatus having an electrode, asponge-like, percolating layer, an electrolyte and an element forapplying a voltage, as defined in the first claim.

In the publication “Porous Nickel Oxide/Nickel Films for ElectrochemicalCapacitors,” J. Electrochem. Soc., Vol. 143 (1996), pp. 124-130,Kuo-chan Liu and Marc A. Anderson report on electrochemical capacitors.The capacitors comprise two layers that are spaced from one another by 7mm and are made of percolating, sponge-like NiO/Ni particles, which areapplied to a carrier and are 3 to 8 nm in size, with a pore size of 2 to3 nm. The layers are disposed in an electrochemical cell that is filledwith a liquid electrolyte. A first circuit, in which the cell and aresistor are connected in series, charges the capacitor via apotentiostat. The discharge is effected by bridging the potentiostat. Amethod for producing the layer is disclosed.

Because the two provided percolating layers are separated from oneanother by a 7 mm-thick layer of the liquid electrolyte, their use islimited to capacitors. Other applications, particularly those based onoptical effects, are precluded. The nature of the interaction betweenthe percolating layers and the liquid electrolyte is not describedexactly; in particular, it is not mentioned whether the liquidelectrolyte penetrates the layers.

“An Electrochemical Route for Making Porous Nickel Oxide ElectrochemicalCapacitors” by Venkat Srinivasan and John W. Weidner, J. Electrochem.Soc., Vol. 144 (1997), L210-L213, describes a further production method.

It is the object of the invention to propose an apparatus having apercolating, sponge-like nanoparticulate layer, the apparatus beingsuitable for numerous technical applications.

The object is accomplished by the apparatus described in claim 1. Thefurther claims disclose preferred embodiments of the apparatus.

In accordance with the invention, an apparatus is proposed that isconstructed on an electrode as the carrier. Suitable electrodes includesolid bodies, films or coatings comprising a highly conductive andchemically inert metal, such as silver, aluminum, nickel, gold, platinumor copper, or a transparent conductive layer, such as ITO (Indium TinOxide). The film is preferably 0.1 to 1000 μm thick. Small filmthicknesses, e.g., in the order of magnitude of 1 μm, are used forelectronic displays; thicker films are used in actuator technology.

The electrode is at least partially covered by a sponge-like,percolating layer comprising bound nanoparticles. “Percolating” is usedhere and hereinafter to refer to a structure that is connectedmechanically, and in an electrically conductive manner, in its entirety.Preferably, only a single percolating layer is provided. Thenanoparticles can comprise a metal such as silver, gold, palladium,platinum, nickel or tungsten, a semiconductor such as silicon, galliumarsenide or cadmium sulfide, or an ionic crystal such as nickel oxide orruthenium oxide.

The maximum size of the nanoparticles is 15 times the size of a formingspace-charge region. The term “space-charge region” is defined andexplained in, for example, K. H. Hellwege, Einführung in dieFestkörperphysik [Introduction to Solid Electrolyte Physics], Springer1976, Ch. 46.2, and Ch. Kittel, Einführung in die Festkörperphysik,Oldenbourg, 10^(th) Ed., 1993, “Schottky Barriers” section. In metals,the space-charge region is about 0.1 nm. Metal nanoparticles shouldtherefore ideally be about 1.5 nm. In semiconductors, the space-chargeregion is between a few and a few hundred nm, depending on the doping.Thus, semiconductor nanoparticles should be about 15 to 3000 nm.

The size of the cavities or pores in the sponge-like, percolating layermust at least correspond to the space-charge region in the electrolyte.They can otherwise be of arbitrary size. To achieve an optimum packingdensity, the size of the cavities is selected to be only slightlygreater than the size of the space-charge region, or about twice thesize of the space-charge region at most.

The cavities of the layer are at least partially filled with anelectrolyte. The electrolyte can be a solid or liquid electrolyte, suchas a salt solution. The cavities should preferably be filled as full aspossible with the electrolyte. Complete filling ensures that the samecharge in the space-charge region in the metal or semiconductor is alsodistributed in the typical space-charge region of the electrolyte.

A film of the electrolyte also borders the free surface of thepercolating layer that is not located on the electrode. This film can beapplied directly to the layer; alternately, a thin, e.g., 0.1 to 10nm-thick, layer of an insulating material can be provided between thelayer and the film. In both cases, the film thickness should be about 1to 1000 μm.

Finally, the apparatus includes an element that can be used to apply avoltage between the electrode and the electrolyte. The voltage should bea few Volts, preferably from 1 to 3 Volts. Voltages of over 10 V areusually not necessary.

In this apparatus, an applied voltage causes electrons to leave theconductivity band of the metal or semiconductor, which forms thesponge-like, percolating layer, and enter the electrolyte. Because ofthe nanostructure of the sponge-like layer, the overwhelming majority ofatoms of this layer are located on easily accessible surfaces, so up to30% of the atoms of the layers can donate electrons. The attained effectcan be utilized in numerous ways.

For example, optical properties can be altered through the applicationand shutoff of the voltage. The apparatus can therefore be used as acolor or black-and-white, large-surface display, for light dimmingcontrol, for automatic brightness adaptation in offices or workrooms, oras a rapid optical modulator. The aforementioned effect also altersmechanical properties, so the apparatus can be used as an actuator or acontrol element with a volume-expansion effect. It is also conceivableto utilize phase transformations occurring with this effect.

The sponge-like, percolating layer can be produced with numerousmethods. Possible production methods include:

a) gas-phase condensation, in which the metal or the semiconductor isvaporized in a vacuum recipient filled with an inert gas at a certainpartial pressure. With suitably selected method parameters, the networkof the sponge-like layer is formed through the collection of theresulting nanoparticles on a cooled substrate;

b) leaching from a mixed crystal or a multiple-phase glass. This processemploys a spinodal separation of an ionic crystal in two phases, e.g., acalcium-rich phase and a barium-rich phase. These two phases arestructured in a sponge-like manner as described above. Because of thevarying solubility, one of the two phases can be dissolved out insuitable solvents, so the entire network is obtained;

c) electochemical separation. Under suitable separation conditions,metals are deposited in a sponge-like layer onto an electrode.

What is claimed is:
 1. An apparatus a) having an electrode; b) having asponge-like, percolating, electrically-conductive layer, which comprisesconnected nanoparticles and contains cavities, and at least partiallycovers the electrode, with the size of the nanoparticles being at most15 times the size of a space-charge region forming in the layer; c) inwhich the cavities of the layer are at least partially filled with anelectrolyte, and are larger than the space-charge region in theelectrolyte; and d) in which the electrolyte also forms a film thatborders the surface of the layer located opposite the electrode; and e)having elements with which a voltage can be applied between theelectrode and the electrolyte.
 2. The apparatus according to claim 1, inwhich a layer of insulating material is provided between the layer andthe film formed by the electrolyte.